To date, commercial filtration of a large quantity of liquid substrate, specifically contaminated water, using immersed fiber membranes, has been prejudiced by the capital cost of construction of a large plant, that is, one for production of at least 3.785 million liters/day (MM L/day) or 1 million gallons per day (MM gal/day) of permeate, and the cost of its operation. To produce permeate efficiently, that is, economically, a skein must be back-pulsed frequently; the more contaminated the substrate, the more frequent the back-pulsing. Because skeins are typically back-flushed several times an hour, and permeate is forced through the fibers during back-flushing during a very short period at a flow rate greater than the rate at which permeate is withdrawn, back-flushing is referred to as "back-pulsing". Fibers may be back-pulsed with water, or gas, typically air, or a mixture of air and water. Filtering municipal sewage, "dirty" river water and the like, may require back-pulsing every 15 min. In a typical commercial filtration plant, a tank is continuously fed with contaminated water; permeate is recovered and concentrate in which the contaminants are concentrated (typically 0.1% to 15% solids) is purged from the tank.
In a conventional filtration plant, multiple modules or skeins are connected for operation in parallel, as if a single skein; and each cassette is provided with an on/off valve on its permeate and back-pulse manifolds respectively, to isolate it from other cassettes, if for example a skein becomes defective and the cassette is to be replaced. On/off valves which are fully open or fully closed are referred to herein because there is no reason to have valves which may be partially opened, and on/off valves are cheaper and operate more quickly. By "isolate" is meant that one or more cassettes is hydraulically disconnected from the permeate collection header for the remaining cassettes. If desired, more than one, but less than all the skeins may be so isolated, at relatively frequent intervals for executing an ancillary function. Functions other than filtration are referred to as "ancillary functions". An ancillary function is typified by back-pulsing with permeate, chemically cleaning with a chemical solution, or integrity testing (to detect a leak referred to as "an integrity test"). To avoid manually turning valves on and off, automatic on/off valves may be used to serve their designed purpose.
Back-flushing with permeate requires flowing it through the lumens of the fibers at sufficient pressure to pass through the pores of the membranes; preferably the pressure is sufficiently high and generated quickly enough to provide a "pulse" which effectively dislodges a build-up of material ("foulant") which may foul and plug the pores. Since skeins are usually back-flushed often, it is desirable to back-flush one skein while the others produce permeate. Though there is no saving in time compared to operating the entire bank as a single skein, since each skein is isolated for the same amount of time irrespective of whether the bank is operated as a single skein, such a procedure allows close control of each skein in the bank. For example, over a 100 cycles, each skein is back-pulsed 100 times. The total time each skein spends being back-pulsed is the same whether all skeins are operated as a single skein or not.
A procedure for simultaneously back-flushing one skein while withdrawing permeate from others in a bank or cluster of skeins is disclosed in German Beschreibung No. 10397/6.11.97 to Prof Chmiel. In his FIG. 1 (reproduced in FIG. 1 herein with valves added) he discloses a tank 4 in which is immersed a cluster of two skeins 2a and 2b of membranes 3 from which permeate is withdrawn at outlet 6. Permeate leaves from the skeins' upper headers through a permeate withdrawal manifold connected to the suction of a permeate withdrawal pump ("permeate pump") 8; their lower headers are connected to the discharge of a back-washing pump ("back-pulse pump"). In this configuration, permeate can only be withdrawn from one direction (from the upper header is shown), and the skein can only back-pulsed in the same direction as permeate is withdrawn, that is by back-pulsing through the lower headers. In a skein with relatively long fibers, that is greater than 0.5 meter, it may be desirable to withdraw permeate from both the upper and lower headers for efficiency; and this cannot be done with the FIG. 1 configuration. Skein 2a is provided with upper and lower automatic on/off valves A1 and A2 respectively, and skein 2b is provided with upper and lower automatic on/off valves B1 and B2 respectively. Thus to operate the two skeins he requires four on/off valves. A machine program switches valving as required to withdraw peremate from one skein while the other is being back-pulsed as follows: when both skeins are in the production mode, valves A1 and B1 are open and valves A2 and B2 are closed; when skein 2b is to be back-pulsed, valve B1 is closed and valve B2 is opened. Back-pulse pump 9 introduces permeate though inlet 5 into the lower header of skein 2b and the permeate is discharged through the membranes at 7 into the water in the tank. After skein 2b is adequately back-pulsed, it is reconnected by opening valve B1 and closing valve B2; simultaneously valve A1 is closed and valve A2 is opened so skein 2a is back-pulsed.
In FIG. 2 (reproduced herein), a top plan view, Chmiel illustrates 8 skeins in a radial cluster, and one skein 2b is back-washed while the other skeins 2a produce permeate. After 2b is back-washed, it is re-connected to produce permeate and another skein is back-washed. As in FIG. 1 each skein has an upper and lower valve (not shown) and permeate is withdrawn from the top header of the skein while back-washing with permeate or chemical cleaning is carried out through its bottom header.
Because the emphasis in Chmiel is on control of each skein, he failed to realize that he may have been able to operate multiple clusters of skeins without valving the skeins, but valving the cluster, and thus save on the number of valves. The possibility of using multiple clusters did not arise because his goal was to control each skein, not to operate a large commercial filtration plant.
A "cassette", like a cluster, comprises plural modules (skeins) each having opposed upper and lower headers, from at least one of which is discharged permeate in open fluid communication with other headers, the skeins being connected for operation in parallel. Conduits from headers of individual skeins in a cassette are not valved. Each skein is in open fluid communication with a cassette manifold. Preferably a cassette has both an upper and a lower CPM which also functions as a manifold for back-pulsing and chemical cleaning. A cassette is a "movable operating unit" which can be installed in a plant without handling individual skeins. A cassette affords no control over a single skein but facilitates handling, installing and operating multiple skeins as if the cassette were a single skein. A cassette has from 2 to about 40 skeins, depending upon the number of fibers in the skein and theri length. A cassette of long fiber skeins preferably has from 4 to 15, and most preferably from 6 to 12 skeins. In a cassette of vertical skeins, each having upper and lower headers, permeate may be withdrawn from both the upper and lower headers, or only one; similarly, a cassette may be back-pulsed through both the upper and lower headers, or only one.
An assembly of cassettes is referred to as a "bank" or "train" of cassettes. A bank of cassettes may be operated as a single cassette. A pump may be used to withdraw permeate from the CPM; the same, or another pump may be used to execute an ancillary function.
Even distributing cassettes evenly on either side of a suction line of a cassette permeate manifold ("CPM"), operation of a bank of from 12 to 16 cassettes as a single cassette, results in such a difference in pressure drops between cassettes connected at opposed ends of the CPM and cassettes near the middle, that it greatly affects efficiency both with respect to permeate withdrawal and also to back-pulsing. The term CPM is used specifically for the permeate manifold; the cassette back-pulse manifold is referred to as "CBPM"; and the two together are referred to as "cassette manifolds". It will be understood that, as used herein, CPM and CBPM refer to manifolds which place each cassette in fluid communication with a permeate pump and a back-pulse pump respectively.
Moreover it was not possible to operate other cassettes in the bank while one was being replaced, so that one was required to wait until the cassette was isolated to re-commence operation with the rest of the bank.